Single mode propagation in fibers and rods with large leakage channels

ABSTRACT

Various embodiments include large cores fibers that can propagate few modes or a single mode while introducing loss to higher order modes. Some of these fibers are holey fibers that comprising cladding features such as air-holes. Additional embodiments described herein include holey rods. The rods and fibers may be used in many optical systems including optical amplification systems, lasers, short pulse generators, Q-switched lasers, etc. and may be used for example for micromachining.

FIELD OF THE INVENTION

This invention relates to optical fiber and optical rods in general andin particular, to optical fiber and rod waveguides with large coredimensions for single mode propagation and devices and systems that usesuch fibers and rods such as lasers and amplifiers.

BACKGROUND

Fiber lasers have demonstrated a great deal of potentials as high powerpulse and cw laser sources, especially for applications where highquality, near diffraction-limited beam is required. Such applicationsinclude precision machining where well defined beam location is criticaland micro-machining and waveguide-writing where a highly focused beam isa useful tool for reaching a threshold power level. The major limitationto the development of fiber lasers with even high peak power isnonlinear effects. The major nonlinear limits are from Raman scatteringand self-phase modulation, although Brillouin scattering can also play arole in narrow line-width laser systems. Nonlinear coefficients are lowfor the silica glass used in most optical fibers. The interactionbetween the low nonlinear coefficients with high peak intensity in thesmall fiber core over a sufficient length can, however, still causesevere pulse distortion and loss of energy. Reduction of fiber length iscertainly one possible approach. This is, however, limited by thesolubility of rare earth ions in the glass host and M² value of themultimode pump lasers. The key to the nonlinear problem is thereforeoptical fibers with large effective mode area while maintaining robustsingle mode propagation. Such fiber is also required to deliver a singlemode beam over distance to the work piece, an important practicalattribute in many applications.

Conventional single mode fiber can, in theory, be adapted to providevery large effective mode area. In practice, such a waveguide is so weakthat the optical fiber becomes very sensitive to its environment,notably bending effects. Single mode propagation in fibers with fewmodes was subsequently proposed (see U.S. Pat. No. 5,818,630 forexample). The robustness of the single mode propagation can bemaintained at reasonable levels in this case especially when care isgiven to ensure single mode launch, minimization of mode coupling andadditional mode filtering. A combination of these techniques has lead toa demonstration of single mode propagation with a mode field diameter(MFD) of ˜30 μm (A. Galvanauskas, “Mode-scalable fiber chirped pulseamplification systems”, IEEE J. Sel. Top. Quantum Electron., 7, 504(2001)). Repeated efforts have also been made in the last few years toprovide a large effective area solution using the emerging photoniccrystal fiber technology. A typical photonic crystal fiber has a regulararray of hexagonally placed air holes surrounding a solid core. Aphotonic crystal fiber supports guided modes in a solid core by provinga composite cladding comprising air holes in a glass background, havinga lower effective refractive index than that of the core. To reduce thenumber of modes in photonic crystal fibers, the state-of-art designemploys small air holes with hole-diameter d to pitch A ratio of lessthan 0.1. In this regime, the photonic crystal fiber is very weaklyguided leading to high environmental sensitivity. Robust single modepropagation in photonic crystal fibers has been limited to a mode fielddistribution (MFD) of ˜28 μm (High-power air-clad large-mode-areaphotonic crystal fiber laser in Optics Express, vol. 11, pp. 818-823,2003), a similar level to that of conventional fiber. This is notsurprising considering the similarity in principle of the twoapproaches. The progress towards fibers with large effective area istherefore relatively stagnant in the past 5-7 years, despite thesignificant progress in fiber lasers.

SUMMARY

One embodiment of the invention comprises an optical fiber forpropagating at least one lower order mode having a wavelength, λ, whilelimiting propagation of higher order modes having a wavelength, λ, byproviding said higher order modes with a higher loss than said at leastone lower order mode at said wavelength, λ, said optical fibercomprising: a first cladding region comprising one or more claddingfeatures; and a core region surrounded by the said first claddingregion, said cladding features configured to substantially confinepropagation of said lower order modes to said core region, said coreregion having a width of at least about 20 micrometers, wherein saidcore region is configured to provide a loss for said higher order modesof at least about 0.5 dB.

Another embodiment of the invention comprises an optical fiber forpropagating at least one lower order mode having a wavelength, λ, whilelimiting propagation of higher order modes having a wavelength, λ, byproviding said higher order modes with a higher loss than said at leastone lower order mode at said wavelength, λ, said optical fibercomprising: a first cladding region comprising one or more claddingfeatures configured to form a partially enclosed region, said partiallyenclosed region having at least one opening therein formed by one ormore spaces in said partially enclosed region, said one or more featureshaving a maximum feature size, d, and a maximum bridge width, a, saidmaximum bridge width in part determining the size of said one or morespaces in said partially enclosed region; and a core region surroundedby the said first cladding region, said cladding features configured tosubstantially confine propagation of said lower order modes to said coreregion, wherein said maximum bridge width, a, and said maximum featuresize, d, have respective values that yield a ratio of a/λ that is atleast about 5 and a ratio of d/λ, that is at least about 10 therebyproviding an increased effective core size, confinement of said at leastone lower order mode, and reduction of said higher order modes.

Another embodiment of the invention comprises a waveguide rod forpropagating at least one lower order mode having a wavelength, λ, whilelimiting propagation of higher order modes having a wavelength, λ, byproviding said higher order modes with a higher loss than said at leastone lower order mode at said wavelength, λ, said rod comprising: a bodycomprising material substantially optically transmissive of saidwavelength, said body having a width and thickness larger than about 250μm; a first cladding region in said body, said first cladding regioncomprising one or more cladding features configured to form a partiallyenclosed region, said partially enclosed region having at least oneopening therein formed by one or more spaces in said partially enclosedregion, said one or more features having a maximum feature size, d, anda maximum bridge width, a, said maximum bridge width in part determiningthe size of said one or more spaces in said partially enclosed region;and a core region in said body, said core region surrounded by the saidfirst cladding region, said cladding features configured tosubstantially confine propagation of said at least one lower order modeto said core region, wherein said maximum bridge width, a, and saidmaximum feature size, d, have respective values that yield a ratio ofa/λ that is at least about 5 and a ratio of d/λ that is at least about10 thereby providing an increased effective core size, confinement ofsaid at least one lower order mode, and reduction said higher ordermodes.

Another embodiment of the invention comprises an optical rod forpropagating at least one lower order mode having a wavelength, λ, whilelimiting propagation of higher order modes having a wavelength, λ, byproviding said higher order modes with a higher loss than said at leastone lower order mode at said wavelength, λ, said optical rod comprising:a first cladding region comprising one or more cladding features; and acore region surrounded by the said first cladding region, said claddingfeatures configured to substantially confine propagation of said lowerorder modes to said core region, said core region having a width of atleast about 20 micrometers, wherein said core region is configured toprovide a loss for said higher order modes of at least about 0.5 dB.

Another embodiment of the invention comprises a hybrid rod structurecomprising: a holey waveguide rod portion comprising at least one coreregion and at least one cladding region, said core region being boundedby air holes surrounding said core region, said air holes furtherlocated within the physical extent of said cladding region; and anon-waveguiding portion connected to said holey waveguide rod portion,said air holes extending along most of the length of the holey waveguideand terminating at the non-waveguiding rod portion.

Another embodiment of the invention comprises a holey waveguide rod,comprising: a ceramic or crystalline laser material; at least one coreregion in said ceramic or crystalline laser material, said core regionbeing bounded by features surrounding said core region; and at least onecladding region in said ceramic or crystalline laser material, saidfeatures disposed within the physical extent of said cladding region.

Another embodiment of the invention comprises a holey waveguide rod,comprising: a ceramic or crystalline laser material; at least one coreregion in said ceramic or crystalline laser material, said core regionbeing bounded by features surrounding said core region; and at least onecladding region in said ceramic or crystalline laser material, saidfeatures disposed within the physical extent of said cladding region.

Another embodiment of the invention comprises a method of manufacturinga holey waveguide rod amplifier, comprising: providing a ceramic orcrystalline laser material; forming at least one air hole configured toform a core region in said ceramic or crystalline laser material, saidcore region being bounded by said at least one air hole, said at leastone air hole comprising a cladding region in said ceramic or crystallinelaser material.

Another embodiment of the invention comprises a waveguide rod amplifier,comprising: a rod having an outside lateral dimension of at least about250 μm across; at least one cladding region; and at least one doped coreregion having a numerical aperture less than about 0.04, said coreregion configured to receive a near diffraction-limited input signal andoutputting an amplified near diffraction-limited output beam.

Another embodiment of the invention comprises an optical fiber forpropagating at least one lower order modes having a wavelength, λ, whilelimiting propagation of higher order modes having a wavelength, λ, byproviding said higher order modes with a higher loss than said at leastone lower order mode at said wavelength, λ, said optical fibercomprising: a cladding; and a core, said core region having a width ofat least about 20 micrometers, wherein said fiber is configured suchthat (i) the at least one lower order mode has no more than 1.0 db ofloss at a bending radius of at 30 centimeters and (ii) said higher ordermodes have a loss of at least 0.5 dB.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are side and cross-sectional views schematicallyillustrating a fiber comprising a core region surrounded by a claddingregion comprising a plurality of cladding features comprising air-holesin a hexagonal arrangement.

FIG. 1C is a plot of wavelength versus maximum feature size (normalizedby center-to-center hole spacing) showing simulation results fordifferent parameter regimes of operation for single mode propagation forin a fiber with 1 and 2 layers of holes in a hexagonal arrangement.

FIG. 2A is a photograph of a cross-section of a multimode holey fiberfabricated to support a single mode and suppress propagation of highermodes.

FIG. 2B illustrates the measured modal field distribution of thefabricated fiber shown in FIG. 2A and the modal field distributionobtained from simulations.

FIG. 3 is a plot showing the measured loss versus bending radius alongtwo bending planes indicated in the inset, a performance feature ofwhich is much improved compared with conventional large mode area fiber.

FIG. 4 is the plot of wavelength versus maximum hole size (normalized bycenter-to-center hole spacing) of FIG. 1A schematically showing thesingle mode operation regime with contour lines of constant bridge widtha/λ.

FIG. 5A us a schematic diagram of a generalized fiber.

FIG. 5B is a cross-section of the fiber of FIG. 5A schematicallyillustrating a core 141 of the fiber 140 defined by features 142, aregion 143 that surrounds 142 the features, a glass bridge width, a,defined as the minimum width of the glass region between the features,and feature size d, which is defined as the width of the feature facingthe core.

FIGS. 6A, 6B, 6C, 6D, 6E and 6F illustrate alternative designs usingnon-circular feature shapes and arrangements.

FIG. 7A schematically illustrates a design for a polarizationmaintaining fiber comprising an asymmetrical core.

FIG. 7B schematically illustrates the incorporation of stress elementsaround the core to create a polarization maintaining fiber.

FIG. 7C is a schematic diagram showing stress elements used incombination with an asymmetrical core and elliptical features.

FIG. 7D schematically illustrates a polarization maintaining fiberdesign using circular features of varying size.

FIG. 7E schematically illustrates a polarization maintaining fiberdesign with stress elements in the core.

FIGS. 8A, 8B and 8C illustrate double clad structures for claddingpumped amplifiers and lasers.

FIG. 9 schematically illustrates fundamental mode excitation in thelarge mode area fiber using a taper at one end of the fiber to reducethe core size so that the fiber is better matched with a single mode orfew-mode fiber. (As shown, a lens can also be used as an alternative toa splice).

FIG. 10 schematically illustrates a preferred arrangement where thefiber is coiled so that unwanted higher order modes can be furtherattenuated by the coils. (A straight section is maintained at the inputend and at the exit end).

FIG. 11 schematically illustrates a large mode area fiber comprising acore doped with rare earth ions that is used as in a fiber amplifier orin a laser pumped by a multimode pump source.

FIG. 12 schematically illustrates an example of how the large mode areafiber can be used in high energy pulse amplification systems.

FIG. 13 schematically illustrates an example of using the large modearea fiber as a delivery fiber for a high power laser system totransport an optical beam to a work piece. (A local lens is used tofocus the beam.)

FIG. 14A is a schematic illustration a configuration where a large corefiber is followed by a length of non-waveguiding fiber so that theguided beam can be substantially expanded through diffraction effectbefore it reaches the glass-air interface.

FIGS. 14B, 14C and 14D schematically illustrate a possibleimplementation of the design in FIG. 14A by collapsing air holes byheating a length of the fiber.

FIG. 14E schematically illustrates a first double clad fiber spliced toa second fiber having a single cladding.

FIG. 15A schematically illustrates a cross-section of an air-clad holeyrod.

FIG. 15B schematically illustrates cross-section of a double clad stepindex rod.

FIG. 15C is a photograph of a single-clad holey rod.

FIG. 15D is a side view schematically illustrating a monolithicwaveguide rod.

FIG. 15E schematically illustrates a cross-section of a waveguide slabcomprising air holes.

FIG. 15F schematically illustrates an exemplary curvilinear waveguide.

FIG. 16 schematically illustrates a high power amplifier systemcomprising a fiber rod and a multimode (MM) pump source.

FIG. 17 schematically illustrates a high power amplifier systemutilizing a fiber rod.

FIG. 18 schematically illustrates a cooling arrangement for a waveguiderod.

FIG. 19 schematically illustrates a generic cw laser configuration usingan ultra-large mode rod.

FIG. 20A schematically illustrates a generic chirped pulse amplificationsystem for femtosecond (fs) or picosecond (ps) pulses utilizing anultra-large mode rod.

FIG. 20B schematically illustrates a generic amplification system fornanosecond (ns) pulses utilizing an ultra-large mode rod.

FIG. 21 schematically illustrates a generic Q-switched waveguide rodlaser.

DETAILED DESCRIPTION OF SOME EMBODIMENTS

Various embodiments of the invention comprise optical waveguides such aslarge core multimode optical fiber. This multimode optical fibercomprises core and cladding regions. The fiber may comprise a matrixmaterial with a plurality of holes formed therein. See FIGS. 1A and 1B.The plurality of holes are in the cladding region and provideconfinement for the core region. The matrix material may comprise, forexample, silica glass and the holes may be air holes.

The core may have a lateral dimension such as a width or diameter.Similarly, the optical fiber itself may have a cross-sectional dimensionsuch as width or diameter as well. The holes may have a lateraldimension, e.g., a cross-sectional size or diameter, d, and an averagecenter-to-center spacing, Λ. Additionally, the holes may be separated bybridges having a bridge width, a. These bridges may comprise the matrixmaterial or material comprising the cladding region.

Large air holes aid in confinement of light in the core region and areparticularly useful for reducing loss induced by bending of the fiber.The high contrast air/glass boundary of the large air holes effectivelyconfines fundamental mode or lower order modes on a bend. Bending losstherefore can be effectively reduced by using larger holes. In someembodiments, for example, bending loss of lower order modes (e.g., 1, 3,5, 10 modes) may be no more than about 1 dB at a bending radius of 30centimeters or more. This leads to designs with smaller number of largerholes for the reduction of bending loss as discussed in further detailbelow.

In some embodiments, equivalent hole diameter-to-pitch ratio, d/Λ, maybe larger than 0.4 in a structure with holes arranged in a triangularpattern. In other embodiments, the ratio of hole diameter to wavelengthd/λ is at least or in excess of about 5. In certain embodiments, valuesof d/λ (where λ is the light wavelength in vacuum) vary between about 10to 100 for core diameters up to about 100 μm. The value of d/λ can beeven larger (e.g., at least about 100) for larger core size (at leastabout 100 μm). Values outside these ranges, however, are possible.

Various embodiments comprise a large core multimode fiber withsufficient built-in mode-filtering to allow robust single modepropagation. Large gaps between the holes provide sufficient leakagechannels for higher order modes, which is useful for maintaining singlemode propagation.

In particular, the maximum width of the glass bridge, a, between theholes that define the core can be used as a design parameter forachieving stable single mode propagation. The value, a, can benormalized to the operating wavelength and the ratio a/λ can be used asa general measurement and design parameter. In one exemplary design tobe described below having six circular holes, a/λ varies from about 5 to40 when core size varies from about 25 to 100 μm at an operatingwavelength of ˜1 μm, for example. For larger core size beyond about 100μm core diameter, for example, larger a/λ, (larger than about 100) canbe used to create larger leakage channels for the higher order modes.Other values, for example, outside these ranges are possible.

The result of these design features is a fiber with a solid coresurrounded by a few large air holes. A large core is provided that canhandle high intensities with reduced nonlinearities and reduced damage.Bending loss can be effectively reduced by using larger holes. Largerbridges may be used to introduce leakage loss to suppress higher ordermodes. Various designs also reduce inter-modal coupling due to highleakage loss of the higher order modes, leading to much improved singlemode propagation.

In various embodiments, hole diameter-over-wavelength ratio is excess ofabout 5. In certain fabricated designs, d/λ, as high as 60 are usedleading to a much reduced bending loss. An effective mode area of 1417μm², an equivalent mode field distribution (MFD) of ˜42.5 μm, has beendemonstrated. Robust single mode propagation has been demonstrated. Thistechnology is expected to have significant impact on the development ofhigh power fiber lasers.

The dimension of the hole facing the fiber core is a particularlyrelevant dimension. Accordingly, features with larger aspect ratio, e.g.long ellipses, with the long dimension facing the core can be usedinstead of circular structure. Varying the shape of the hole can providemore flexibility in some designs.

Controlling the bridge width, a/λ, also allows the designer moreflexibility in creating fiber designs especially when other constraintsneed to be considered. Non-circular holes, varying number of holes,non-regularly distributed holes can all be used in a design. Inaddition, different materials as well as different configurations mayalso be used as well.

Asymmetrical geometry, for example, either in distribution of holesor/and shape of holes can be used to create polarization maintainingfibers. Stress-inducing elements can be incorporated in holes in anasymmetrical fashion to create polarization maintaining effect as well.

The holes can, in general, be of any shape. Additionally, the holes maybe filled with substantially optical transparent materials, for example,having refractive index lower than the rest of the matrix material(which may comprise, e.g., glass). The holes do not have to be uniform.The number of holes can be as low as one.

Rare earth ions, e.g. ytterbium and erbium, can be incorporated into thecore to form an active medium. Accordingly, gain can be provided in afiber when pumped with appropriate pump sources.

A pump guide can be further incorporated around the core region and thefeatures used to define the core. A double clad design may be used. Pumpenergy can be injected into the pump guide to pump active ions in thedoped core. This pump guide may comprise an additional layer around thecore region and the features used to define the core. This additionallayer may have an effective lower refractive index either by using,e.g., a low index polymer coating or air-hole structures comprising ofmainly air and a small amount of glass in some embodiments discussedmore fully below. In case of using air hole structures to form the pumpcladding, an additional glass layer around the pump cladding may be usedto provide structural support. A polymer coating may be used as theoutermost layer to provide further protection.

A taper can be formed at one end of the large core to provide a fiberend which is either single mode or has fewer modes. This end havingreduced size can be used for either splicing to a single-mode orfew-mode fiber or launching light into the fiber. Stable excitation offundamental mode in the large core fiber can readily be achieved usingthis type of taper.

The field distribution in a mode in the large core fiber can be modifiedby bends even when minimum power loss occurs. This effect is due to acombination of weak guidance in the large core and larger stress-inducedrefractive index change over the much larger core. The optical waveguideis sufficiently modified by bends, which leads to a change in mode fielddistribution. In various preferred embodiments, however, when the largecore fiber is used, both the launching end and exit end are straight toachieve better excitation of the fundamental mode and a desirable outputmode profile.

One application of the large mode area fiber is for high power laserdelivery. The much reduced nonlinear effect in the large core fiberallows much higher power to be carried by the fiber. The large mode areafiber can also be potentially used as a compressor in a high energychirped pulse amplification system, where optical pulses are chirped,reducing their peak power in amplifiers, before being compressed back totheir original pulse width. This compressor function can also beintegrated into an amplifier and/or delivery fiber for someapplications. The low nonlinearity of the fiber also allows this largecore fiber to support low order solitons with much higher peak power.This feature can be useful in some applications. Accordingly, in someembodiments, the fiber may be coupled to a source of solitons.

In addition to higher order mode filtering, reduction of inter-modecoupling also improves robust single mode propagation in a large corewaveguide thereby reducing or minimizing power transfer from thefundamental mode to higher order modes. Increasing the fiber diameterreduces the mode-couplings, which in turn also allows the core diameterin the fiber to be increased (see, e.g., U.S. Pat. No. 5,818,630). Inthe extreme case fiber rods are so obtained. Such fiber rods also reduceor minimize mode coupling due to a reduction of micro-bending. Such rodhave enough rigidity to maintain their physical shapes. Rods can bedeployed in a in a straight configuration and alternatively, fixed bentconfigurations when appropriate. Recently, such fiber rods weredescribed by N. Deguil-Robin et al. in “Rod-type fiber laser”, AdvancedSolid State Photonics, 2005. The fiber rod concept was further developedby Limpert et al., in “High-power Q-switched ytterbium-doped photoniccrystal fiber laser producing sub-10 ns pulses” Conf. on Advanced SolidState Photonics, paper PD-1, Vienna (2005).

The design of optimized multi-mode ultra large-core fiber structures maypermit near diffraction limited outputs. Suppression of higher ordermodes may be particularly useful in producing near diffraction limitedoutputs. These multi-mode ultra large-core fibers may have corediameters, for example, of at least about 15 μm and outside diameters atleast about 200 μm to reduce or minimize mode-coupling. Such ultra-largecore fiber structures can resemble rod structures that are sufficientlyrigid so as not allow for any significant bending or coiling (unlesspre-bent). Fiber structures based on conventional step-index as well asholey fiber designs can be implemented.

Both step-index as well as holey fiber designs can further incorporatedoped core regions to facilitate their use as optical amplifiers orlasers. By incorporating a double cladding into the ultra large corefiber amplifier and laser structures, cladding pumping e.g., withsemiconductor laser diode arrays is possible. Alternatively, pump lightcan be directly coupled into the core region of the ultra large corefiber structure.

The reduction of mode-coupling in such ultra large-core fiber structuresfurther allows direct core pumping with multi-mode laser beams whilepreserving a near diffraction limited output for the amplified lasermode.

For the specific case of high energy ytterbium amplifiers, multi-modepump sources based on Nd or also Yb fiber lasers can be implemented.Alternatively, a frequency doubled Tm fiber laser can also be used forcore-pumping of an ultra large-core Yb fiber laser. Other configurationsand designs are also possible.

Moreover, ultra large-core fiber structures can be combined with taperstructures (e.g., at the pump end) to further increase the output peakpower from these systems. The taper structures are preferably pumpedwith near-diffraction limited pump beams, though conventional lowbrightness pump sources can also be implemented. Alternatively taperstructures can also be used to simplify input coupling into these largemode fibers.

Ultra large mode fiber rods can also be designed to follow curvilinearpaths to reduce or minimize actual storage space for such structures.Such curvilinear paths can be obtained by gently heating sections of thefiber rod and bending it into a desired shape. Fiber rods formed intocurvilinear shapes further allow for the introduction of differentiallosses between the fundamental and higher order modes.

The construction of laser systems based on three level transitions isfurther facilitated by the implementation of multi-mode holey orconventional step-index fibers based on ultra-large cores. In case thereis competition between three and four energy level systems such as inytterbium doped fibers, the four-level system often lases first atlonger wavelength due to the low inversion required. This is especiallytrue for long ytterbium-doped fiber length, where any emission at theshorter wavelength of the three energy level system is absorbed to pumpthe four energy system. In a double clad fiber with a large core, wherepump can be absorbed over a short length, a shorter ytterbium-dopedfiber can be used and facilitates the laser action from the three energysystem. These laser systems may, for example, be used as pump sourcesfor ultra-large core fiber amplifiers and fiber tapers.

Ultra-large core fibers and rods further allow the amplification ofultra-short pulses via the chirped pulse amplification techniques.Ultra-large core fibers and rods also allow Q-switched operation as wellas frequency conversion to the UV and IR. Such pulse sources, forexample, can generate peak powers of at least about 1 MW and pulseenergies of several mJ for pulses with a width of only about 1 ns.

Further the construction of holey large mode waveguides is not limitedto fiber materials, holey large mode waveguides can also be constructedin ceramics, plastics and crystalline materials. In some embodiments,these structures are uniformly doped and can be directly core pumped orcladding pumped. Waveguide rods or slabs allow extraction of much largergains from active materials compared to standard laser rod technology.Undoped waveguide structures based on ceramics, plastics and crystallinematerials can also be envisioned.

As used herein, single mode and multimode fiber are defined consistentlywith the definitions used for traditional non-holey fiber. Fortraditional fibers, single mode and multimode fiber are generallydefined in terms of V number, which is equal to π (numerical aperture)(core diameter)/wavelength for step index fibers. For non-step indexfibers, numerical aperture and core diameter can be calculated with theequivalent values for step index fibers [see, e.g., Martinez, F., Husey,C. D., “(E)ESI determination from mode-field diameter and refractiveindex profile measurements on single-mode fibres” IEEE Proceedings V135,pp. 202-210, (1988)]. For fibers satisfying the relationship V<2.4, thepower of the fundamental mode is significantly larger than the opticalpower of the next higher mode. Alternatively, for fibers wherein V>2.4,at least the next mode above the fundamental mode can have significantpower in comparison to the fundamental mode. Single mode and multimodetraditional fibers are accordingly specifically defined by therelationships V<2.4 and V>2.4, respectively. V=2.4 is the cut-off forthe propagation of any mode but the lowest order mode.

In holey fibers, the numerical aperture can be found by the differencein refractive index of core and cladding. However, a core diameter thatis the equivalent value for step index fibers is difficult to calculate.Various references [see, e.g., (1) Knight et al, “Properties of photoniccrystal fiber and the effective index model” J. Opt. Soc. Am. A Vo. 15,pp. 748-752, (1998), and (2) Mortensen et al “Modal cutoff and the Vparameter in photonic crystal fibers” Opt. Lett. V. 28, pp. 1879-1881,(2003)] report that if the core diameter is made equal to the pitch orthe distance between holes, Λ, then the V for cut off for thepropagation of any mode other than the single mode is 2.5 (see, e.g.,Knight et al) and π (see, e.g., Mortensen et al). For the variousembodiments described herein, whether the V cut-off is 2.405, 2.5 or πis not critical. Various embodiments of holey fiber described hereinhave a much larger core radius than possible with conventional opticalfiber that supports propagation of a single optical mode. Therefore, wewill utilize the recent research in this technical area where multimodefiber is defined as where V>π and the core diameter is made equal to thepitch or average pitch to the fiber. Conversely, single mode fiber isdefined herein as fiber where V<π.

As described above, holey fiber may be designed to introduce loss forspecific modes. The hole size, bridge, and the number of holes may, forexample, be selected to induce loss in the propagation of higher ordermodes in a multimode fiber where V>π. With a decrease of the number ofholes, light in the higher order modes may not be confined to the coreand may escape from the fiber. Such loss introduced into multimode fiberV>π is analogous to traditional non-holey multimode fiber having a Vnumber larger than π that include mode filtering provided, for example,by bending the fiber to introduce loss in the propagation of higherorder modes. (Mode filters are described in, e.g., U.S. Pat. No.5,818,630 issued to Fermann et al on Oct. 6, 1998 and entitled“Single-mode Amplifier and Compressors Based on Multi-mode Fibers,”which is hereby incorporated herein by reference.) Sufficient bendingcan be applied to induce losses for each of the modes higher than thefundamental mode such that the fundamental mode is the sole mode thatpropagates through the bent multimode fiber. Similarly, multimode holeyfiber having a V number larger than about π may have a design thatintroduces loss to higher order modes so that propagation of thesehigher order modes is attenuated. See, e.g., U.S. patent applicationSer. No. 10/844,943 filed May 13, 2004 and entitled “Large Core HoleyFibers” (Attorney Docket No. IMRAA.024A), which is incorporated hereinin its entirety.

In various designs discussed below, therefore the maximum size of thecladding features, e.g., air holes, the maximum bridge width, the numberof layers (e.g., 1 or 2), may be such that only a few (e.g., 3, 5, 10)lower order modes, or even only a single mode propagates without muchloss while higher order modes propagate with much greater loss.

FIGS. 1A and 1B schematically illustrate one embodiment of a holey fiber500. A cross-section of the fiber shown in FIG. 1A is presented in FIG.1B where different parameters are defined. As shown, d is hole diameterand Λ is center-to-center hole spacing. Core radius, ρ, is the distancefrom the center to the nearest hole-boundary. The values can benormalized to wavelength, λ, the wavelength of light in vacuum.

FIG. 1C is a plot of wavelength versus maximum feature size thatillustrates simulation results for different parameter regimes ofoperation for single mode propagation labeled 502 and 501 for a holyfiber such as shown in FIGS. 1A and 1B with 1 and 2 layers of holes in ahexagonal arrangement. The upper boundaries 503 and 505 of 501 and 502in FIG. 1C are determined by maximum tolerable loss of the fundamentalmode and the lower boundaries 504 and 506 by minimum propagation loss ofthe second order modes. The plot in FIG. 1C shows that with a reductionof the number of holes from 2 layers to 1 layer, d/Λ moves towards largevalues for the same core size. The contour lines of constant corediameter 2ρ are shown in lines 507, 508 and 509 for core diameter of 25,50 and 100 μm respectively. FIG. 1C is computed for straight fibers. Forbent fibers as it may be in a practical case, the operation regimes 501and 502 are moved towards larger d/Λ. In FIG. 1C, 1OM refers tofundamental mode and 2OM second order mode. Core size refers to corediameter 2ρ and equals to 2Λ-d in the hexagonal case illustrated in FIG.1B.

FIG. 2A is a photograph of an exemplary fiber 510 fabricated and tested.In particular, FIG. 2B, 511 shows the measured modal field distributionof the fabricated fiber 510. Also shown in the respective modal fielddistribution 512 calculated from a model of the fiber 510 shown in FIG.2A. FIG. 2B also provides a plot 513 and curve 514 that show themeasured field profile through the center of the mode along therespective y and x axes, which are delineated in FIG. 2A. FIG. 2B alsoprovides a plot 515 and curve 516 shows the respective modeled fieldprofile. Single mode operation is clearly demonstrated by thedistributions. This fiber 510 supports single mode propagation with ameasured effective modal area of about 1400 μm², which is obtained fromtaking appropriate integration of the measured mode field distributionin 511.

FIG. 3 illustrates the measured loss versus bending radius along twobending planes that are indicated in the inset. The bend loss of thefiber was measured by winding the fiber on mandrels of known diameters.Since the cross-section of the fiber lacks rotational symmetry, thedependence of the bend loss on the orientation of the bending plane wasinvestigated. Specifically, as shown in the inset of FIG. 3, a bendingplane AA that intersects with two smaller holes is defined. Similarly abending plane BB that intersects with two thin glass ridges is alsodefined. The output of the fiber is imaged onto a video camera with anaspheric lens. By monitoring the output beam profile, single modepropagation can be ensured throughout the measurement. The amount ofbend loss in decibel per meter is plotted in FIG. 3 as a function of thebend radius in centimeters. Bending along the plane AA introduces lessloss in the fiber as compared to bending along the plane BB, which canbe explained by the presence of the two small holes helping to betterconfine the mode. As can be seen in FIG. 3, the amount of bend loss indecibels per unit length as a function of bend radius follows the samefunctional dependence

$\left\lbrack {= {\frac{\alpha}{\sqrt{R_{bend}}}{\exp\left( {{- \beta}\; R_{bend}} \right)}}} \right\rbrack$that was developed for conventional optical fibers, where R_(bend) isthe bend radius. The fitting parameters are α=3755 dB·cm⁰⁵/m, β=1.2581/cm for bending along the plane AA, and α=2.265×10⁴ dB·cm⁰⁵/m, β=1.4601/cm for bending along the plane BB.

FIG. 4 shows the same single mode operation regimes plotted in FIG. 1Cfurther including contour lines of constant bridge width, a/λ. Operationregimes 121 and 120 are enclosed by boundaries 124, 125 and 122, 123 arefor 1 layer and 2 layers of holes respectively. The contour lines 130,131, 132, 133, 134 and 135 of constant bridge width are respectively forbridge widths, a/λ=1, 2, 4, 10, 20 and 40. For core diameter from about25 to 100 μm, a/λ varies from about 5 to 40.

FIGS. 5A and 5B illustrate a generic fiber 140. A Core 141 in fiber 140is defined by features 142 in a cladding region. Region 143 furthersurrounds the features 142 in the cladding region. Glass bridge width,a, is defined as the minimum width of the glass region between holes.Feature size d is defined as the width of the feature.

FIGS. 6A, 6B, 6C, 6D, 6E, and 6F illustrate alternative designs usingnon-circular features in various arrangements. FIG. 6A shows a pluralityof elliptical holes 151 in a circular arrangement that define a centralcore region 150. The holes 151 are formed in a matrix material 152. FIG.6B shows a plurality of differently shaped holes 155 in an irregulararrangement that define a central core region 154. The holes 155 areformed in a matrix material 156. FIG. 6C shows circular holes 158arranged in a non-circular pattern that define a central core region157. The holes 158 are formed in a matrix material 159. As used herein,features may comprise holes that include air or are evacuated and,hence, contain vacuum. Additionally, these holes may be filled withanother material, for example, different material than the matrixmaterial to form the features. FIGS. 6D, 6E and 6F further described howa and d are defined in various designs. The fibers FIGS. 6D and 6E havea pair of holes while the fiber in FIG. 6F has a single hole.

FIG. 7A illustrates a design for a polarization maintaining fiber.Elliptical features 162 surround a generally elliptical or rectangularcore 161. The use of long ellipses enhances the level of birefringenceand reduces bending loss. The features are in a matrix material 163which surrounds the features. In FIG. 7B, stress elements 164 are usedaround the core 165 to create a polarization maintaining fiber. The core165 is further defined by additional features 166. In FIG. 7C, stresselements 168 are used in combination with an asymmetrical core 167 andelliptical features 169. FIG. 7D illustrates a polarization maintainingfiber design with circular features. Two of the features aligned along aplane are reduced in size to provide asymmetry for maintainingpolarization. FIG. 7E illustrates a fiber design 400 where stresselements 414 and 415 are introduced in the core 401 defined by sixcircular features 403 in a hexagonal arrangement. The fiber 400 can be adouble clad design by introducing a low refractive index pump cladding404 surrounding the pump guide 402. An outer layer 405 is also provided.

FIGS. 8A and 8B illustrate a double clad structure for cladding pumpedamplifiers and lasers. FIG. 8A illustrates a core 170 defined by thefeatures 171 defining an inner cladding, which is in turn surrounded byouter cladding 172. Pump cladding 173 is a material with a lowerrefractive index, for example, a polymer coating. However, a secondglass with lower refractive index can also be used. FIG. 8B showsanother double clad fiber comprising a core 174, which is defined byfeatures 175 defining an inner cladding, and a surrounding outercladding 176. In FIG. 8B, a pump cladding 177 comprising air holes andthin glass bridges (not shown) is used. A further glass region 178surrounds the pump cladding 177 to provide mechanical support. Althoughnot shown, thin glass bridges connect the outer cladding region 176 andthe glass region 178. A polymer coating surrounding glass region 178,not shown in FIG. 8B, may be also applied to a fiber. In FIG. 8C, thepump cladding 179 is shown to be a rectangular shape. In fact, this pumpcladding can take any shape around the pump guide. Accordingly, otherconfigurations are possible.

FIG. 9 illustrates two exemplary configurations for fundamental modeexcitation in the large mode area fiber 180. In the first configuration,a taper is made at one end of fiber 180 to reduce its core size so thatit is better matched with a single mode or few-mode fiber 181. Lightcoming from the single mode or few-mode fiber 181 excites substantiallyonly the fundamental mode in the large mode area fiber if good modematching between the two fibers is achieved with the taper. In a secondconfiguration shown in FIG. 9, a lens 183 is used to launch thefundamental mode into the fiber 180 through the taper 182 as analternative to a splice.

FIG. 10 illustrates an exemplary arrangement where the fiber is coiled190 so that any unwanted higher order modes can be further attenuated bythe coils. A straight section 191 is maintained at the input end. Astraight section 192 is also maintained at the exit end. Modal fielddistribution can be distorted on bends in the large mode area fiber dueto weak guidance and relatively stronger stress-induced refractive indexchange. The straight sections ensure better coupling into thefundamental mode and thus better launching of a fundamental mode as wellas good output mode profile. A lens 193 is shown coupling light into thestraight section of the fiber at the input end. The coil 190 can also beused to maintain polarization along the fiber. The birefringence is dueto stress optics effects as a result of asymmetry in bend-inducedstress. This bend-induced stress is larger in a large core fiber.

FIG. 11 shows a large mode area fiber 200 with a core doped with rareearth ions used in a fiber amplifier or a laser pumped by a multimodepump source. The fiber has straight input and output ends 201, 202 and acoiled section therebetween. A multimode pump 205 is used to pump theamplifier or laser using a coupling lens 204. Input beam 206 is launchedinto fiber 200 through a lens 203. Output 207 is separated by dichroicmirror 208.

FIG. 12 illustrates an example of how the large mode area fiber can beused in high energy pulse amplification system. Optical pulse from aseed source 210 is stretched in the pulse stretched 211. Pulse picker212 reduces the pulse repetition rate. A large core amplifier 213comprising the large mode area fiber amplifies the pulse and the pulseis then compressed using a pulse compressor 214 close to its originalwidth in output beam 215.

FIG. 13 illustrates an example of use of the large mode area fiber as adelivery fiber 221 for high power laser system 220 to transport opticalbeam 222 to work piece 224. A local lens 223 is used to focus the beam222. A positioning system may be used to position the output beam withrespect to the work piece. This positioning system may comprise atranslation stage, for example, on which the work piece is disposed.Movable optics such as a movable mirror or lens may be used. Otherconfigurations and designs are also possible.

In FIG. 14A, a fiber 240 comprises a first length comprising a coreformed by cladding 241, followed by a second length without a core. Abeam 242 propagates from the first length to the second length where thebeam expands in size due to diffraction. The beam 242 is substantiallyexpanded when the beam reaches the fiber end where damage threshold islow. This can prevent end face damage at the output end of an amplifierwhere power is the highest.

The configuration of FIG. 14A can also be implemented by splicing thelarge core fiber with another coreless fiber. This approach isespecially useful when the length for expanding the beam 242 is long(e.g., from several centimeters to tens of centimeters long).

FIGS. 14B, 14C and 14D illustrate an implementation of the fiberstructure of FIG. 14A. A fusion splicer was used to collapse holes alonga length of the fiber to form the coreless section. FIG. 14B shows across-section of the first length where the holes are present and FIG.14D a cross-section of the second length where the holes are collapsedand not thus present.

The first double clad fiber 600 in FIG. 14E comprises a first cladding602, defining a core and a second cladding 603, defining a pump guide.The first fiber 600 is then spliced to a second fiber 601. The secondfiber 601 has a single cladding 604, which has a dimension close to thatof second cladding 603 in first fiber 600. The second fiber 601 can beused to expand a beam 605 and may also be used for pump power (not shownin FIG. 14E) to be launched into second fiber 601.

As described above, in various preferred embodiments the size of thefiber is large such that the fiber effectively becomes a rod that isrigid. FIG. 15A comprises an exemplary holey fiber design 250 such asdiscussed above. The various designs and concepts described herein withreference to fibers are also applicable to rods.

In FIG. 15, the holey fiber or rod 250 comprises a core region 251 whichis preferably index matched to the cladding region 252. The core region251 can also have other another index, such as a refractive index lowerthan cladding region 252. In various preferred embodiments, the coreregion 251 is further doped with rare-earth ions such as Yb, Nd, Er orTm to provide gain. Appropriate glass-forming dopants such as B or F areadded to the glass to obtain a doped core area index matched (or with adepressed refractive index compared) to the cladding area 252. Inside ofcladding 252, a set of air-holes 253 are arranged that define the coreregion 251. As discussed above, the separation of the center of theair-holes Λ may be close to the diameter of the air-holes d, e.g.,d/Λ>0.4 to obtain an improved or optimum mode-quality for thefundamental mode. However, other values, for example, ratios of d/Λ<0.4can also be used. In various preferred embodiments, the fiber cansupport more than two core modes. Moreover, it can also be desirable toadd additional holes to the cladding region 252 to reduce the claddingarea. The glass ridges between the air-holes can be at least a fewwavelengths in width, i.e. in the range of 5-100 μm, such that pumplight from the cladding region 252 penetrates into the core region 251.Air-cladding region 254 surrounds cladding region 252. As discussedabove, the air-cladding region can comprise an array of very thin glassridges that are arranged around cladding region 252 and connect tooutside cladding region 255. Cladding region 254 may be designed with alow effective index correspondingly producing a large acceptance angle(or numerical aperture NA) for pump light coupled into cladding region252. In various preferred embodiments, the outside diameter of fiber 250is at least about 250 μm to reduce or minimize mode-coupling.

A simpler fiber rod structure 260 based on a more conventional fibergeometry is shown in FIG. 15B. The fiber rod 260 in FIG. 15B comprises astandard rare-earth-doped step-index core gain region 261 and a claddingregion 262. To confine pump light in cladding region 262, a low indexregion 263 is further added. Since fiber rods are generally keptstraight and mode-coupling is substantially reduced or minimized with alarge outside fiber diameter, core NAs in the range from 0.01-0.05 canstill be effectively used in the present example. Refractive indexvariations in the core region are kept to a minimum in various preferredembodiments. To provide a large index difference between claddingregions 262 and 263, multi-component glasses such as Ta or Sb-dopedsilica glasses can be used. Such glasses are discussed in Dejneka etal., U.S. Pat. No. 6,836,607. Using such multi-component glasses aneffective NA for the pump guiding region of about 0.40 or higher can beobtained.

Hybrid forms of the designs shown in FIG. 15A and FIG. 15B can also beemployed for ultra large mode fiber rods. For example, an air-cladding265 can be incorporated into cladding area 262 for the construction ofan air-clad conventional step-index fiber.

A photograph of a cross-section of a fiber rod manufactured according tothe design principles discussed above is shown in FIG. 15C. Thisexemplary fiber rod has a core diameter of 58 um and a cladding diameterof 270 um. The air-holes have a diameter of around 40-46 μm. Noair-cladding 254 was incorporated. With appropriate launching conditionsa fundamental mode with a mode-field diameter of 42 um could be coupledinto this fiber. The fiber was manufactured with the stack and drawtechnique. Note that this rod further includes holes of differentdiameter (namely 40 and 46 μm) along two axes of the fiber so as toinduce a degree of form birefringence into the structure. Note thatbirefringence can further be introduced into the fiber by coiling,allowing for polarization maintaining operation.

The various designs discussed in connection with FIGS. 15A-15C can beutilized in the rod 101 used in the amplifier and laser systems shown inFIGS. 16, 17, and 19, which will be discussed in more detailed below.

As an example, consider the amplifier system 100 depicted in FIG. 17. Inthis system 100 the fiber rod amplifier 101 is connected and preferablyfused to a rod amplifier 102. Fiber rod 101 is constructed with a doubleclad structure, comprising an outside cladding 103, an inner cladding104 and a rare-earth doped core region 105. In certain embodiments, theinner and outside cladding are not doped, though doped claddingstructures can also be implemented. The rod structure 102 comprises anunpumped region 106 and a pumped region 107. This rod structure 102 canbe uniformly doped with a gain medium just like a conventional laserrod. Amplifier rods 101 and 102 are pumped with pump sources 108 and109, respectively. In certain preferred embodiments, pump source 108 ismulti-mode and pump source 109 is single-mode. However, efficientamplifiers can also be constructed based on a single-mode pump source108; equally pump source 109 can also be multi-mode.

Appropriate pump sources can be based on near diffraction limited ormulti-mode fiber or solid state lasers, as well as beam-shapedsemiconductor lasers. Such beam-shaped semiconductor lasers aredescribed, e.g., in U.S. Pat. No. 6,778,732 issued to Fermann et al. Twolenses are used to couple the pump sources into the rod amplifierstructures 101 and 102. Single lenses are shown only for simplicity.More generally, appropriate lens systems can be used for pump coupling.Dichroic beam splitters 110 and 111 are used to combine/separate thepump sources with the input and output signals 112, 114, respectively.The input signal 112 is preferably near diffraction limited and injectedinto fiber rod structure 101. The direction of the pump light from pumpsource 109 is denoted with arrow 113. The output 114 from the wholeamplifier system is also denoted with a larger arrow.

A direct comparison can be made with the features of the rod depicted inFIG. 15A and the rod shown in FIG. 17. For example, core region 251corresponds to core region 105; cladding region 252 corresponds tocladding region 104 and cladding region 254 corresponds to claddingregion 103. A separate outside cladding region 255 was not shown in FIG.17. The simple design shown FIG. 15B also has its exact equivalence inthe design of fiber rod 101 as shown in FIG. 17.

Optionally, fiber 101 in FIG. 17 (as well as in FIGS. 16 and 19,discussed more fully below) can be surrounded with a polymer jacket orit can be metalized and fixed to a heat sink for efficient thermal heatdissipation. In various preferred embodiments, the air-holes insidefiber 101 are thermally collapsed at the signal input end to avoid endface contamination and to reduce or minimize damage to the structure.Additionally, the signal input end of fiber rod 101 can be tapered downto a smaller diameter to enable single-mode propagation and tofacilitate excitation of the fundamental mode in fiber rod 101.

Several specific examples of different fiber designs and rod designs arepresented below.

FIBER DESIGN EXAMPLE 1

This design includes six air holes arranged in a hexagonal shape asillustrated in FIG. 1A. The center-to-center spacing, Λ, is 40 μm, thehole size 30 μm, yields a core diameter 2ρ of 50 μm for operation at ˜1μm wavelength. The bridge width a/λ is 10 and the normalized hole size,d/λ, 30.

FIBER DESIGN EXAMPLE 2

This design includes six air holes arranged in a hexagonal shape asillustrated in FIG. 1A. The center-to-center spacing, Λ, is 80 μm, thehole size 60 μm, yields a core diameter 2ρ of 100 μm for operation at ˜1μm wavelength. The bridge width a/λ is 20 and the normalized hole size,d/λ, 60.

FIBER DESIGN EXAMPLE 3

This design includes six air holes arranged in a hexagonal shape asillustrated in FIG. 1A. The center-to-center spacing, Λ, is 160 μm, thehole size 120 μm, yields a core diameter 2ρ of 200 μm for operation at˜1 μm wavelength. The bridge width a/λ is 40 and the normalized holesize, d/λ, 120.

FIBER DESIQN EXAMPLE 4

This design includes six air holes arranged in a hexagonal shape asillustrated in FIG. 1A. The center-to-center spacing, Λ, is 40 μm, thehole size 30 μm, yields a core diameter 2ρ of 50 μm for operation at ˜1μm wavelength. The bridge width a/λ is 10 and the normalized hole size,d/λ, 30. Two stress elements comprising boron-doped silica areincorporated in two diagonally opposing air holes to produce apolarization maintaining fiber.

For all the design examples describe herein, the actual fibercross-sections are usually different from performs due to viscous flowduring the fiber drawing process in cases performs are made according tothe designs. Frequently, fiber preforms are slightly modified fromdesigns to conform any practical constraints. Also, other sizes andmaterials may be used. For example, glass other than fused silica, e.g.phosphate, fluoride, telluride, lead silicate, etc, can be used. Infact, phosphate glass may allow higher rare earth doping levels. Asdiscussed above, the air holes can be replaced by one or more materials,for example, glass with lower effective indexes in all design examples.Rare earth ions or a combination of rare earth ions, such as ytterbium,erbium, thulium, neodymium, etc., can be doped in the core region toprovide gain. Double clad structure can also be implemented to providean outer pump guide such as depicted in FIG. 8. Other dimensions andconfigurations can also be used.

ROD DESIGN EXAMPLE 1

This design comprise a structure similar to that shown in FIG. 15A. Therod can be constructed from silica glass providing core absorption of600 dB/m at a wavelength of 980 nm, corresponding to an ytterbium dopinglevel of around 1 weight %. The air-holes can have a diameter of 40 μmand the core diameter (defined as the closest separation betweenopposite air-holes) can be 50 μm. The inner diameter or air-cladding 254can have a diameter of 150 μm. The outside diameter can be anywhere inthe range from about 250 um to about 10 mm or even higher. The NA of theair-cladding can be 0.6. Hence a high average cladding absorption of 65dB/m at a wavelength of 980 nm can be achieved. The main reason for theimprovement in cladding absorption is the improved core design of thestructure.

ROD DESIGN EXAMPLE 2

This design comprise a structure similar to that shown in FIG. 15B.Again the rod can be constructed from silica glass providing a coreabsorption of 600 dB/m at a wavelength of 980 nm. The core diameter canbe 50 μm and the inner diameter or the air-cladding 264 can have adiameter of 150 μm. The core NA can be 0.04 and the NA of theair-cladding can be 0.6. Again an average cladding absorption of 65 dB/mat a wavelength of 980 nm can be achieved. An outside fiber diameter ofabout 250 μm to about 10 mm and larger can be used.

With currently available pump sources, up to about 100 W can be coupledinto the above two fiber structures, enabling the generation ofamplified average powers at least about 50 W for fiber length less thanabout 50 cm. Gains of about 30 dB or higher can further be achieved fromsuch fiber structures in lengths less than about 50 cm.

Using such rods in a short pulse amplification system, for an effectivemode diameter of about 50 μm, 1 ns long pulses with a pulse energy up to2.5 mJ can be generated, limited by the bulk damage threshold of silica.

Even higher pulse energies can be generated by using rod-section 102 inFIG. 17. Mode-expansion in rod-section 102 is governed by diffraction.For a mode diameter, ω, of about 50 μm, the Rayleigh range R can bedefined over which diffraction leads to a mode expansion by a factor of√2 as R=nπω²/2λ, where n is the refractive index, λ is the operationwavelength and w is defined as the diameter between the points where themode intensity is reduced by a factor of 1/e compared to the intensityat the center of the mode. For a mode diameter, ω, of about 50 μm andn=1.5 (for silica glass) at a wavelength of λ=1 μm, the Rayleigh range,R, is about 6 mm. Ensuring optimum mode overlap between pump source 109and the output from fiber rod 101, an increase in mode size by a factorof 4 (from about 50 to 200 um) can be obtained in a doped rod 102 of 24mm length. For a fully inverted rod gain medium 102 made from the samecore material as fiber rod 101, a gain of 2 dB/cm can be achieved at thepeak of the ytterbium gain band at 1030 nm. Thus, a 24 mm length rod canincrease the maximum pulse energy by around 5 dB. When a high powersingle mode (SM) pump source 107 is available, the same pump source canbe used to pump rod 102 and fiber rod 101.

An even better situation can be obtained when using a fiber rod with a100 um core diameter. In this case, a 42 mm long rod is used to obtain amode-size of 200 um. Hence, a gain of 8 dB can be obtained in rod 102.Eventually, the achievable gain in rod 102 is limited by thermallensing. However, appropriate designs of fiber rod 101 and rod 102 canensure a reduced or minimal of mode distortions even for high gainvalues in rod 102. Moreover, the thermal lens can itself be used toprovide a degree of waveguiding. In addition, gain-guiding effects asdiscussed in Fermann et al., '630 can be used to improve the modequality at the output of fiber rod 102. To minimize thermal distortionsdue to end effects in rod 102, an additional undoped rod (not shown) canbe fused to the output end rod 102.

The fiber rod/rod combination may benefit from efficient externalconvective or conductive cooling. Conductive cooling may provide higherheat dissipation. A possible conductive cooling arrangement is shown inFIG. 18. Here structure 300 comprises the fiber rod/rod combination 301,which can be soldered into metal holder 302 in certain embodiments.Metal holder 302 is cooled by 4 water channels 303, 304, 305, 306arranged centro-symmetrically around rod 301. More or less waterchannels can be used. A uniform a temperature profile can be obtained byproper designs. Other configurations are possible.

The use of rod 102 in FIG. 17 for maximization of available pulseenergies is purely optional and fiber rod 101 can be used by itself togenerate high pulse energies such as shown in FIG. 16 discussed morefully below. When using fiber rod 101 (without rod 102), small angles(not shown) can be introduced at the fiber ends to avoid parasiticreflections in such structures. Small undoped rods can be fused to fiberrod 101 as end-caps to enable mode-expansion and to increase or maximizethe damage threshold of the fiber rod ends.

The achievable gain per unit length in rod 102 can further be optimizedby the use of multi-components glasses, such as Bi- or phosphateglasses, ceramic or crystalline materials such as Yb:Y₂O₃ or Nd:YAGrespectively, which can be fused directly to the glass rod. Here Bi- andphosphate glass, Yb:Y₂O₃ ceramic or Nd:YAG are cited only as examples,and in principle any active gain medium can be used for rod 102. Gainmedia such as Nd:YAG, Nd:YLF or Nd:YVO₄, Nd:glass, Yb:glass, Nd:KGW,Yb:KGW and Yb:KYW are further examples of structures that can be shapedinto fiber rods. Theses gain media can be uniformly doped.

More generally, such holes can be directly incorporated into uniformlydoped glasses, ceramic or crystalline materials such as Bi- andphosphate glasses, Yb:Y₂O₃ or Nd:YAG and even Ti:Sapphire as well asplastics such as PMMA. Most glasses such as Bi-glasses and phosphate aswell as some ceramic materials can be drawn into fibers and thestructure as shown in FIG. 15A can then be simply drawn using the wellknown stack and draw technique. Alternatively, appropriate holes can bemicro-machined into a crystal using precision mechanical drilling, laserablation, or ultra fast optical pulses. The holes in fiber rods can bemuch larger compared to holey fibers. To reduce or minimize surfaceirregularities in the actual hole structure and to increase or maximizethe Rayleigh range of the ablating laser, UV pulses can be used.

When employing uniformly doped materials as fiber rods or generally aswaveguide rods, the use of direct pumping into the core structure withnear diffraction limited pump sources is useful for achieving a goodoverlap between pump and signal beam.

When creating holes in generic amplifier media, such as glass, plastics,ceramics or crystalline materials, monolithic designs of hybrid versionsof waveguide rods can be constructed. The side-view of a such amonolithic hybrid waveguide rod 220 is shown in FIG. 15D). Waveguide rod220 comprises a uniformly doped gain material 221 and a propagationregion 222. Propagation region 222 is bounded by symmetrically arrangedair holes 223 and 224 (e.g., six large, tightly spaced holes, may beused as shown in FIG. 15A) on one side (left side of drawing) for modallight confinement, whereas the propagation region is allowed to diffractfreely on the opposite side (right side of drawing). The structure canbe pumped from the freely propagating region (right side of drawing).The signal is injected into the confined propagation region (left sideof drawing). Holes 223 and 224 can be made by mechanical drilling orthey can be constructed using laser ablation. In such structuresfundamental mode sizes up to about 100 um or larger can be obtained. Toavoid contamination of air-holes 223 and 224, the input and output endfaces are preferably polished and antireflective (AR) coated prior tothe introduction of the air-holes. The input and output end faces canalso be tilted or wedged (not shown) in order to avoid parasiticreflections in the amplifier.

Since particularly crystalline or ceramic waveguide rods can be quiteshort due to manufacturing restraints or because the doping level inthese structures may be limited due to thermal and efficiencyconsiderations, the signal in such waveguide rods can be multi-passed toincrease the achievable gain. Referring back to FIG. 15D, such waveguiderods operated in a multi-pass configuration are preferably constructedwithout a freely diffracting region. Standard methods for multi-passesthrough waveguides incorporating Faraday rotators and polarization beamsplitters can be implemented to obtain a double or a quadruple passthrough waveguide-rods.

Due to thermal considerations, it can be advantageous to constructwaveguide slabs instead of waveguide rods. A generic implementation of awaveguide slab 230 is shown in FIG. 15E. This slab is wider than it isthick. Exemplary dimensions may range from about a width of 250 μm to 10mm. Here the core region 231 is derived from a substantially uniformlydoped active material 232, such as a glass, a ceramic, or a crystal. Theelongated core structure is defined by eight air-holes. The structure ispreferably directly core-pumped. A predominantly one dimensional heatflow can be obtained by attaching heat sinks and appropriate coolingmechanisms to the top and bottom of slab 230. Appropriatemode-transforming optics can then also be used to transform theelliptical output beam into a more circular beam as required for manyapplications. Moreover, cladding pumping can be enabled by surroundingthe substrate 232 with a material with a lower refractive index thansubstrate 232. To avoid scattering losses due to pump light directedinto the air-holes, the air holes in the slab structure can be collapseddown at the pump coupling end.

In order to save storage space for long waveguide rods and to induce adifferential loss between the fundamental mode and any higher-ordermodes, it may be advantageous to coil the rigid structure up into rigidcoils or other structures following curvilinear paths. Because of thesmall bend loss of large mode area holey waveguide rods, curvilinearpaths can be introduced without any major performance limitations.Moreover, curvilinear paths are beneficial for a discrimination of themodes within the waveguide, because of the introduction of bend lossesfor the higher-order modes. An example of a glass or ceramic waveguiderod following a curvilinear path is shown in FIG. 15F. Even relativelyrigid waveguide rods of up to 1 mm (or larger) outside diameter can beformed into arbitrary shapes by gently heating the rods and bending theminto the desired shape.

Though the design examples listed up to this point comprised mainlywaveguide rod structures with substantially circularly shaped air-holes,for the operation of such waveguides, the exact shape, number, andarrangement, of the air-holes may vary. Designs illustrated in FIGS. 5A,5B, 6A-6F, 7A-7E and 8A-8C, for example, can all be used as a rodstructure.

Polarization maintaining operation of waveguide rods can be obtained byconstructing these structures in birefringent crystalline materials andexciting a major axis of the crystal. For the case of fiber rods, stressregions can be incorporated into the fibers to enable polarizationmaintaining operation. Examples of a polarization maintaining fiber roddesigns are shown in FIG. 7A-7E.

Since mode-coupling in fiber rods is greatly reduced, multi-mode (MM)pump sources can be coupled into the core region of the fiber rod withsmall or minimal effects on the beam quality of the amplifier outputbeam. Near diffraction limited beam output are possible. Such anarrangement is shown in FIG. 16. The system is very similar to the onedepicted in FIG. 17. However, rod 102 is eliminated and no separateair-cladding is needed in rod 101. SM pump 109 is substituted with MMpump 115. Pump 115 can comprise a few-mode fiber laser or a highbrightness semiconductor laser. Also shown is a fiber taper 116 whichfacilitates excitation of the fundamental mode in fiber rod 101. Due tothe absence of mode-coupling (or very small mode-coupling in fiber rod101) the fundamental mode can propagate with minimal distortions infiber rod 101 despite its MM structure. The MM structure, however,accepts MM pump beams. For example for a 50 um core diameter fiber rodas depicted in FIG. 15A, a MM pump beam containing 5-20 modes can beused to pump the core area of fiber rod 101. For a 100 μm core diameterfiber rod, up to 100 pump modes can be supported. Hence, an efficientbrightness converter is obtained with the present configuration.

The structure depicted in FIG. 16 can also be used in conjunction withwaveguide rods and slabs as shown in FIG. 15A-15C. Not all waveguiderods may allow for the incorporation of tapers 116 as shown in FIG. 16.Hence, particularly for crystalline waveguide rods and slabs, anon-tapered waveguide 101 may need to be implemented.

The ultra-large core amplifier fibers and rods may be use as anefficient cw fiber laser operating on three-level transitions, such asNd-fibers operating at about 920 nm, or ytterbium fiber lasers operatingat about 980 nm. An exemplary set-up of an efficient Nd fiber laseroperating in the range between about 920-940 nm wavelength is shown inFIG. 19. Fiber rods are not necessary and long-lengths of holey fiberssuch as shown in FIG. 15A may be used. Such long lengths of holey fibercan incorporate mode-filters to facilitate cw lasing in the fundamentalmode of the holey fiber structure, or lasing in at least the few lowestorder modes. Moreover, the fiber can be coiled onto a drum to enablepackaging of the device. Even a few-mode cw fiber laser can be used aspump source for a fiber rod as explained with respect to FIG. 16. Systemin FIG. 19 is slightly modified in comparison to the system shown inFIG. 16. The system in FIG. 19 includes a taper 116 as a mode filter anda fiber grating 117 to obtain preferential lasing on the 940 nm lasingtransition of Nd fiber when pumping in the 800 nm wavelength region.(See, e.g., Fermann et al. in U.S. Pat. No. 5,818,630). A dichroicmirror 118 is further optionally directly deposited onto the pumpcoupling end of the fiber 101. The holes in the pump coupling fiber endcan be collapsed and subsequently polished to obtain a smooth end faceand to improve or optimize pump coupling efficiency as well as tosimplify the deposition of optional mirror 118. Alternatively the flatpolished pump coupling end can be used as a reflective structure.

An exemplary embodiment can comprise a holey fiber with a 60 um core anda 250 um cladding diameter. Fiber lengths between 1 to around 30 m canbe optimally employed. Even with a standard cladding NA of 0.45 asachievable with polymer clad fibers, greater than about 100 W of pumppower can be coupled into the fiber cladding. For a cladding NA of 0.60,pump power up to about 200 W can be coupled into the fiber cladding withconventional high brightness pump sources, enabling the generation of upto 100 W of power near 940 nm. Such near diffraction-limited high powercw sources are ideal for direct core pumping of ytterbium fiber rods ingeneral and specifically uniformly ytterbium doped fiber rods, which donot comprise any undoped cladding region.

Similar design consideration also hold for the design of ytterbium fiberlasers operating at 980 nm, which are even better pump sources fordirect core pumping of ytterbium fiber rods. Generally, the ultra-largecore fibers as discussed here can be used for the demonstration oflasing on any three-level transition in rare-earth doped fibers.

The waveguide rod amplifiers as discussed here are also advantageous foruse as power amplifiers for compact high energy amplification systemsfor ns, ps, and fs pulses. A generic system for the amplification of fsor ps pulses, based on the chirped pulse amplification technique isshown in FIG. 20A. System 300 comprises a seed source 301, an opticalgate 302, a waveguide rod amplifier 303 as well as a pulse compressor304. The output from the system and the direction of light propagationis designated with arrow 305. In some embodiments, seed source 301 maycomprise a laser that produces femtosecond or picosecond pulses, a pulsestretcher, and several pre-amplifier stages. Such systems are describedin U.S. patent application Ser. No. 10/992,762 Fermann et al, filed Nov.22, 2004 entitled “All-fiber chirped pulse amplification system” (DocketNo. IM-114) which is incorporated herein by reference in its entirety.The optical gate 302 may comprise an optical isolator and anacousto-optic modulator to reduce or minimize any amplified spontaneousemission generated in the seed source from coupling into waveguide rodamplifier 303. Compressor 304 may be dispersion matched to thecompressor within the seed source to enable the generation of theshortest possible pulses at the output from the system. Some degree ofdispersion mismatch between stretcher and compressor is tolerable whenexploiting nonlinear pulse propagation of the stretched pulses,particularly when enabling the generation of cubicon pulses; see, U.S.patent application Ser. No. 10/992,762 referenced above. This approachis particularly useful for the generation of femtosecond pulses. Thesystem can be designed to generate cubicon pulses already in seed source301 or it can also be designed for cubicon pulses formation in waveguiderod amplifier 303. Cubicon pulse formation may be enabled when injectinghighly stretched optical pulses into an amplifier and amplifying them toa peak power that subjects the pulses to significant levels ofself-phase modulation. Optional frequency conversion stages can furtherbe implemented down-stream from system 305. When amplifying ps pulses,pulse stretchers as well as compressor 304 can be omitted. Moreover,nonlinear spectral compression as discussed in U.S. patent applicationSer. No. 10/927,374, filed Aug. 27, 2004, and entitled “High-energyoptical fiber amplifier for ps-ns pulses for advanced materialprocessing applications”, (Docket No. IM-105) which is incorporated byreference herein, can be used to obtain near bandwidth-limited ps pulsesat the output. Nonlinear spectral compression can be induced into apositive dispersion waveguide rod amplifier by injecting negativelychirped pulses and amplifying the pulses to a peak power that subjectsthem to significant levels of self-phase modulation. Otherconfigurations are also possible.

A generic nanosecond (ns) pulse amplifier is shown in FIG. 20B. Thenanosecond amplifier 306 is very similar to the system 300 shown in FIG.20A, however, no stretcher and compressor are used. Also shown is anoptional frequency conversion element 307 that can typically comprise anonlinear crystal or an array of nonlinear crystals employed forfrequency up- or down-conversion. Nanosecond as well as picosecond typefiber (or rod) amplifiers can further be used as pump sources foroptical parametric amplifiers, allowing for the generation of pulseswith widths less than about 50 fs and even less than about 10 fs. Suchoptical parametric amplifier systems are discussed in U.S. patentapplication Ser. No. 11/091,015 entitled “Optical parametricamplification, optical parametric generation and optical pumping inoptical fiber systems” filed Mar. 25, 2005 (Attorney Docket No.IMRAA.026A), which is incorporated herein by reference in its entirety.

When generating ns pulses it is most cost effective to employ active orpassive Q-switching. Such a system configuration is shown in FIG. 21.The system 100 is very similar to the system already described withrespect to FIG. 16. However, an optical modulator 118 is added in frontof one of the cavity mirrors (in this case, the fiber Bragg grating117). Two additional lenses 119, 120 (or more generally a lens system orcoupling system) are used to couple light from the tapered fiber rodoutput end 116 to the mode-filter 117. A multi-mode fiber rod 101 can beused as the active gain element. Near diffraction-limited operation ofthe system is obtained by the mode-filtering action of taper 116 inconjunction with fiber Bragg grating 117. More generally, waveguide rodscan also be implemented in this system configuration. Since the pulseenergy generated in such systems can exceed a few mJ, a mode-filterbased on a spatial filter can be used at one end of the cavity.Alternatively, taper 116 can be eliminated and fiber 117 can be replacedwith a single-mode air-holed fiber to provide mode-filtering action.Mode-filtering can also be obtained by shaping the waveguide rod 101into a curvilinear form. A mirror can then be incorporated at one end ofthe air-holed fiber to provide a second reflective structure for theconstruction of a cavity. Such cavity implementations are not separatelyshown.

The ultra-large mode fibers and rods as discussed above are particularlysuitable for a range of machining and marking applications comprisingmachining of metals, ceramics, glasses, semiconductors, crystals,biological systems and others just to mention a few examples. Since theabove systems allow the generation of pulses with energies up to a fewmJ at average powers of tens of W, very high throughput for lasermachining becomes possible. The outputs may also be nearly diffractionlimited. Such systems may include positioning systems, such astranslators for translating the work piece or movable optics (e.g.,movable mirrors or lenses, etc.). Other configurations are possible.

Moreover, various embodiments of the invention have been describedabove. Although this invention has been described with reference tothese specific embodiments, the descriptions are intended to beillustrative of the invention and are not intended to be limiting.Various modifications and applications may occur to those skilled in theart without departing from the true spirit and scope of the invention asdefined in the appended claims.

1. An optical fiber for propagating lower order modes, having order N orless and having a wavelength, λ,while limiting propagation of all higherorder modes with order greater than N by providing said all higher ordermodes with a higher loss than said lower order modes at said wavelength,λ, said optical fiber comprising: a first cladding region comprising aplurality of lower-index cladding features, each of said plurality oflower-index cladding features having a refractive index lower than arefractive index of said first cladding region, said plurality oflower-index cladding features arranged in no more than two layers; and acore region surrounded by said first cladding region, said core regionsubstantially surrounded by each layer of said no more than two layers,said plurality of lower-index cladding features configured tosubstantially confine propagation of said lower order modes to said coreregion, said core region having a width of at least about 20micrometers, wherein said order N is less than or equal to 20, andwherein said core region is configured to provide a loss for said allhigher order modes of at least about 0.5 dB.
 2. The optical fiber ofclaim 1, wherein said core region is configured to provide a loss forsaid all higher order modes of at least about 1 dB.
 3. The optical fiberof claim 1, wherein said width of said core region is at least about 40micrometers.
 4. The optical fiber of claim 1, wherein said plurality oflower-index cladding features has a minimum size, d, yielding a ratio ofd/λ that is at least about
 10. 5. The optical fiber of claim 4, whereinsaid ratio of d/λ is at least about
 20. 6. The optical fiber of claim 1,wherein said plurality of lower-index cladding features are arranged ina single layer.
 7. The optical fiber of claim 1, wherein said core andcladding regions are in a substantially optically transmissive main bodyof said optical fiber, said main body comprising material substantiallyoptically transmissive of said wavelength, λ, said main body having awidth and thickness at least about 200 μm so as to reduce mode couplingof said lower order modes to said higher order modes.
 8. The opticalfiber of claim 7, wherein said width and thickness of said main body isat least about 500 μm.
 9. An optical amplifier comprising an opticalfiber according to claim 1, wherein said core region is doped to provideoptical gain.
 10. A system comprising the optical amplifier of claim 9,wherein output pulse energy is in excess of 1 μJ.
 11. An optical systemfor generation of high power solitons comprising the optical fiber ofclaim
 1. 12. A laser system comprising the optical fiber of claim 1,wherein said optical fiber is doped with dopants that provide saidoptical fiber with gain and said optical fiber is optically coupled toreflective elements that form an optical resonator.
 13. A fiber laseraccording to claim 12, wherein said fiber comprises a Nd doped fiber.14. A fiber laser according to claim 12, wherein said fiber comprises aytterbium doped fiber.
 15. A chirped pulse amplification system for theamplification of picosecond or femtosecond pulses comprising the opticalamplifier of claim 9 and a pulse compressor.
 16. A laser micro-machiningsystem comprising the chirped pulse amplification system of claim 15 anda positioning system.
 17. An optical fiber according to claim 1, whereinthe core and cladding regions comprise polymer materials.
 18. Theoptical fiber of claim 1, wherein said order N is equal to
 1. 19. Theoptical fiber of claim 1, wherein said order N is less than or equal to3.
 20. The optical fiber of claim 1, wherein said order N is less thanor equal to
 5. 21. The optical fiber of claim 1, wherein said order N isless than or equal to
 10. 22. The optical fiber of claim 1, wherein saidfirst cladding is discrete.
 23. The chirped-pulse amplification systemof claim 15, wherein said compressor comprises a fiber, and wherein saidamplifier comprises a portion of a transmission fiber or a portion ofsaid compressor or both.
 24. The optical amplifier of claim 9, whereinsaid amplifier is a pulse amplifier to amplify picosecond, femtosecond,or nanosecond pulses.
 25. The laser system of claim 12, furthercomprising an optical modulator disposed within said resonator to causeactive or passive q-switching of said laser.
 26. The laser system ofclaim 12 wherein said laser system is a CW system operating in thefundamental mode or few lowest order modes.
 27. The optical fiber ofclaim 1, wherein at least one layer of said no more than two layerscomprises a ring.
 28. The optical fiber of claim 1, wherein at least onelayer of said no more than two layers comprises two or more of saidplurality of lower-index cladding features.
 29. The optical fiber ofclaim 28, wherein said at least one layer comprises six or more of saidplurality of lower-index cladding features.
 30. The optical fiber ofclaim 1, wherein said plurality of lower-index cladding features arearranged in two layers.
 31. The optical fiber of claim 30, wherein eachof said two layers comprises a plurality of lower-index claddingfeatures.
 32. An optical rod for propagating lower order modes havingorder N or less and having a wavelength, λ, while limiting propagationof all higher order modes with order greater than N by providing saidall higher order modes with a higher loss than said lower order modes atsaid wavelength, λ, said optical rod comprising: a first cladding regioncomprising a plurality of lower-index cladding features, each of saidplurality of lower-index cladding features having a refractive indexlower than a refractive index of said first cladding region, saidplurality of lower-index cladding features arranged in no more than twolayers; and a core region surrounded by the said first cladding region,said core region substantially surrounded by each layer of said no morethan two layers, said plurality of lower-index cladding featuresconfigured to substantially confine propagation of said lower ordermodes to said core region, said core region having a width of at leastabout 20 micrometers, wherein said order N is less than or equal to 20,and wherein said core region is configured to provide a loss for saidall higher order modes of at least about 0.5 dB.
 33. The rod of claim32, wherein said core region is configured to provide a loss for saidall higher order modes of at least about 1 dB.
 34. The rod of claim 32,wherein said width of said core region is at least about 40 micrometersacross.
 35. The rod of claim 32, wherein said features have a minimumfeature size, d, yielding a ratio of d/λ that is at least about
 10. 36.The rod of claim 32, wherein said core and cladding regions are in asubstantially optically transmissive main body of said rod, said mainbody comprising material substantially optically transmissive of saidwavelength, λ, said main body having a width and thickness at leastabout 250 μm so as to reduce mode coupling of said lower order modes tosaid higher order modes.
 37. The optical fiber of claim 32, wherein saidorder N is equal to
 1. 38. The optical fiber of claim 32, wherein saidorder N is less than or equal to
 3. 39. The optical fiber of claim 32,wherein said order N is less than or equal to
 5. 40. The optical fiberof claim 32, wherein said order N is less than or equal to
 10. 41. Theoptical fiber of claim 32, wherein said first cladding is discrete. 42.The optical rod of claim 32, wherein at least one layer of said no morethan two layers comprises a ring.
 43. The optical rod of claim 32,wherein at least one layer of said no more than two layers comprises twoor more of said plurality of lower-index cladding features.
 44. Theoptical rod of claim 43, wherein said at least one layer comprises sixor more of said plurality of lower-index cladding features.
 45. Theoptical rod of claim 32, wherein said plurality of lower-index claddingfeatures are arranged in a single layer.
 46. The optical rod of claim32, wherein said plurality of lower-index cladding features are arrangedin two layers.
 47. The optical rod of claim 46, wherein each of said twolayers comprises a plurality of lower-index cladding features.
 48. Anoptical fiber for propagating lower order modes having order N or lessand having a wavelength, λ, while limiting propagation of all higherorder modes with order greater than N by providing said all higher ordermodes with a higher loss than said lower order modes at said wavelength,λ, said optical fiber comprising: a cladding further comprising aplurality of lower-index cladding features, each of said plurality oflower-index cladding features having a refractive index lower than arefractive index of said cladding, said plurality of lower-indexcladding features arranged in no more than two layers; and a core, saidcore substantially surrounded by each layer of said no more than twolayers, said core having a width of at least about 20 micrometers,wherein said order N is less than or equal to 20, and wherein said fiberis configured such that (i) the lower order modes have no more than 1.0dB of loss at a bending radius of about 30 centimeters and (ii) said allhigher order modes have a loss of at least 0.5 dB.
 49. The optical fiberof claim 48, wherein said order N is equal to
 10. 50. The optical fiberof claim 48, wherein said order N is equal to
 5. 51. The optical fiberof claim 48, wherein said order N is equal to
 1. 52. The optical fiberof claim 51, wherein the fundamental mode has no more than 0.5 dB ofloss at a bending radius of 10 centimeters.
 53. The optical fiber ofclaim 48, wherein at least one layer of said no more than two layerscomprises a ring.
 54. The optical fiber of claim 48, wherein at leastone layer of said no more than two layers comprises two or more of saidplurality of lower-index cladding features.
 55. The optical fiber ofclaim 53, wherein said at least one layer comprises six or more of saidplurality of lower-index cladding features.
 56. The optical fiber ofclaim 48, wherein said plurality of lower-index cladding features arearranged in a single layer.
 57. The optical fiber of claim 48, whereinsaid plurality of lower-index cladding features are arranged in twolayers.
 58. The optical fiber of claim 57, wherein each of said twolayers comprises a plurality of lower-index cladding features.